Alex and Mark are walking home from the park when Alex brings up a topic in school that he didn't understand.
Today's lesson was confusing. I didn't understand how to add and subtract fractions with unlike denominators.
It's actually pretty simple. Let me tell you. Let's try to solve five sevenths plus three fourths.
You know how to add fractions with common denominators? You have to add the numerators and keep the denominators the same. Here, you have to find a common denominator between the two fractions you have, so when you do, you just have to add the numerators.
Okay. So how to find the common denominator of 4 and 7?
To find a common denominator, you have to find a multiple, which is a number that you get when you multiply your starting number with another number, that is divisible by both numbers and make them that number.
So the common multiple, or denominator, of 7 and 4 is 28.
Now that you have a common denominator that is bigger than your original number, you have to multiply the numerator by the same number you did for the denominator, then you add.
So, we multiply both the 3 and 4 by 7 to get twenty-one twenty-eighths, and we multiply 5 and 7 by 4 to get twenty twenty-eighths. 20 plus 21 is forty-one twenty-eighths, and that turned into a mixed fraction is 1 thirteen twenty eighths! What about subtraction?
To subtract fractions, you do the same thing as adding. you find the common denominator but instead of adding, you subtract. What is three fifths minus one third?
The common denominator of 3 and 5 is 15. 3 times 3 is 9 and 1 times 5 is 5, so the fractions now are nine fifteenths minus five fifteenths. 9-5 is 4, so the answer is four fifteenths.